Future downtown buildings would grow dramatically taller under a proposal Mayor Greg Nickels will unveil today as part of a plan to create a denser and more vibrant urban core.

Although many support that goal, concerns are being raised about blocked views, loss of affordable housing, and whether developers are being asked to provide enough public benefits, such as open space.

"I think residents definitely want investment in their communities, but at the same time not everyone wants to be crammed in with tall structures," said Laine Ross, chair of the Downtown Seattle Residents Council, a neighborhood advocacy group.

Nickels said yesterday the proposed changes grew from a yearlong and inclusive discussion about what people wanted from their downtown. It borrows tools being used in Vancouver, B.C., and San Diego to create thriving center city neighborhoods, he said.

Nickels has proposed doubling the number of downtown residents over the next two decades to 66,000, in part to lessen growth pressure on single-family neighborhoods and curb suburban sprawl.

"We've got good job density downtown, we have a very healthy retail and cultural life, and we think if we add a significant number of new residences then we will have a downtown that will be the rival of any in the United States," Nickels said.

The proposed zoning changes that will be sent to the City Council this week would allow skyscrapers in the city's commercial core to rise to 700 feet -- slightly shorter than the Washington Mutual Tower.

It would wipe out a cap on downtown building heights that residents -- fed up with a slew of office towers -- voted to impose in 1989.

In parts of the Denny Triangle, where planners envision more office buildings, heights could double to 600 feet. In areas where the city would like to encourage more housing, height limits will be expanded to 400 feet.

The changes don't apply to the retail core, Pioneer Square, the International District and most of Belltown. The mayor dropped plans to raise height limits along the waterfront south of Pike Place Market after residents complained.

Jim Mueller, a principal for Gregory Broderick Smith Real Estate, said the current height restrictions have made some projects economically unfeasible. They've also created shorter, stocky buildings that architects and residents alike have complained are unattractive.

Under the mayor's proposal, the taller buildings would be more slender and spaced farther apart. Proponents say that helps preserve view corridors and allows for more light and air between buildings.

It also tweaks a program requiring developers to provide public benefits if they want to maximize profits.

For the first time, developers who go above a certain height limit would be required to build "green buildings" with environmentally friendly features, such as rainwater collection systems or recycled building materials.

Residential developers would now have to contribute to an expanded program that builds affordable housing, potentially increasing that pool of money by $35 million over the next 15 years.

Heather Trim, who works on urban issues for the environmental group People for Puget Sound, said the public should get the amenities that make dense neighborhoods work: open space, nice pedestrian features, greenery, transit options.

She said the city had made some improvements, such as providing enticement for developers to build showers for bike commuters. The proposal also revamps open space requirements to encourage public areas on the street level rather than rooftop gardens or private balconies.

But she said there are concerns that the incentives still won't provide enough money to buy enough open space downtown and that the city lacks a coherent plan to acquire it.

"We want the whole package -- we don't want half of the package," she said.

John Fox, who heads the Seattle Displacement Coalition, which advocates for low-income residents, panned the proposal as a blueprint that will lead to the destruction of low-income housing in favor of high-priced condo towers. But Sarah Lewontin, executive director for the non-profit housing developer Housing Resources Group, said the changes will provide more affordable housing in the long run.

It will expand the city's housing incentive program that contributed a critical $1.3 million toward the group's Stewart Court project, which provides 65 apartments for lower-income workers. That's one of the few funding sources for housing aimed at workers earning a few dollars more than minimum wage, she said. "Otherwise, the only people who can afford to live downtown will be really rich people and really poor people," she said. "It's a very difficult problem, so ... why turn our backs on an opportunity that will help create affordable housing?"

Nickels said he expects a fair amount of debate on whether the proposal offers the right mix of public benefits. But with an extensive involvement process, he said, it represented a good start toward something that "everybody can buy into."

He also said the proposed zoning changes are just part of a larger effort to create a more livable downtown, which includes sprucing up parks and working on safety issues.

Elle Tracy, who has lived in Pioneer Square for 21 years, complained that the mayor has been responsive to developers' problems while dragging his feet on quality-of-life issues that downtown residents have been complaining about for years.

"We would love to have more neighbors, because it seems like we can't get anyone's attention from the city when it comes to things like a noise ordinance and enforcing the laws we have," she said. "But the only beneficiary that we can see as we look at the fine print are developers."

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